Biomedical Engineering Reference
In-Depth Information
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(
A
)
Regions of
equal energy
Figure 1.20
The effect of pulse stacking on skin overlying leg vein. Two previ-
ous pulses caused no effect, then immediate whitening was noted. Biopsy
would show full-thickness necrosis (treatment was with neodynium:yttrium-
aluminum-garnet laser).
Pulsewidth
time is short compared with the time for pressure relaxation.
Here the laser-induced pressure causes compressive stresses
in tissue. Microcracks in the tissue are the result of these large
stress gradients (27).
Damage
threshold
Large
Small
Time
Thermal Injury to Cells
There is a range of measurable effects on skin based on tem-
perature. Below 43°C, the skin remains unharmed, even for
very long exposures (4). Thus one can exceed body temperature
by about 5°C without a measurable change in the skin. The fi rst
change is a conformational change in the molecular structure.
These typically occur at temperatures from 42°C to 50°C. After
several minutes, there will be tissue necrosis as described by the
Arrhenius equation (an equation that quantitatively describes
conversion of tissue from a native to denatured state). Thermal
denaturation is a rate process: heat increases the rate at which
molecules denature, depending on the specifi c molecule. For
example, at 45°C, cultured human fi broblasts die after about
20 minutes. However, the same cells can withstand over 100°C
for 10
−3
seconds (32). In general, a temperature of >60°C lasting
for at least 6 seconds leads to irreversible damage (4).
(
B
)
Figure 1.19
(
A
) Target-size selectivity by choice of pulse width. (
B
) Relation-
ship between size of target and pulse duration to peak temperature. The lon-
ger pulse favors heating of the larger target (blood vessel) versus the smaller
melanosome.
Thermokinetic Selectivity
Along the same lines is the concept of thermokinetic selectiv-
ity (TKS). In this model, one selects larger or smaller targets
based on pulse duration (Fig. 1.19A,B). For example, if one
wants to heat larger targets while sparing relatively smaller
ones, the pulse duration is extended beyond the thermal
relaxation time of the smaller target. In this manner, that is, a
melanosome will be heated to a lower temperature than the
subjacent vessel.
reaction types
Photothermal
Most laser applications in dermatology rely on heating. Tem-
perature is directly related to the average kinetic energy of
molecules. As temperature is raised, tissue coagulates (19). A
familiar example of denaturation and coagulation is the cook-
ing of an egg white. Thermal denaturation is both temperature
and time dependent, yet it often shows an all or none like
behavior. For a given heating time, there is usually a narrow
temperature region above which complete denaturation
occurs. This is readily evident in Figure 1.20. As a rule, for
denaturation of most proteins, one must increase the temper-
ature by about 10°C for every decade of decrease in the heating
time to achieve the same amount of thermal coagulation (19).
Photothermal processes depend on type and degree of heat-
ing, from coagulation to vaporization. A mild to moderate tem-
perature increase results in breakage of hydrogen bonds and
van der Waal bonds leading to denaturation of enzymes and
function. If the heating is very fast, a phase change occurs (27).
Depending on the rate of energy delivery, photovaporization
occurs with or without inertial confi nement (vide infra), where
Coagulation
This is normally the fi rst heating step that is identifi able on rou-
tine light microscopy. An absolute temperature for coagulation-
denaturation does not exist. It appears that for very short times,
higher temperatures than the often-quoted “62-65°C” would
be required. It is not known to what extent the Arrhenius for-
mulation holds for very short heating times (less than 1 second)
(33). In most scenarios of light-tissue interaction, tissues and
cells may be reversibly or irreversibly damaged. Normally,
pathologists examine tissue for changes such as vacuolization,
hyperchromasia, and protein denaturation (birefringence loss).
Moderate temperature-induced damage phenomena in tissue
have been diffi cult to assess with conventional methods of
detection, such as light microscopy. However, particularly in
light of newer large volume low-intensity heating devices for
rejuvenation, more sensitive tools might be required to charac-
terize subtle thermal effects. Beckham et al. (33) found that
over a narrow temperature range, heat shock protein (HSP)
expression correlated with laser-induced heat stress, and that
the HSP production followed the Arrhenius integral. Thus HSP
